This is why we can have ***urance even though we stumble.
"Blessed be the God and Father of our Lord Jesus Christ, who according to His abundant mercy has begotten us again to a living hope through the resurrection of Jesus Christ from the dead, to an inheritance incorruptible and undefiled and that does not fade away, reserved in heaven for you, who are kept by the power of God through faith for salvation ready to be revealed in the last time." 1 Peter 1:3-5

We are kept by the power of God through faith for salvation, just as we are saved by grace through faith. In both instances if there is no faith there is no salvation. After we are saved we must remain faithful.